The most direct move, preparing to capture on c4 and also Na4.
13. b2b4  13... c6c5 Black must play this before White can play c4-c5. The game is quickly heading toward total equality without any imbalances. But we'll still go through a little adventure to get there. 14. b4c5  14... b6c5  15. c2b2 Removing the queen from the c-file, leaving Black's queen in a potential attack by the rook on c1. But this isn't dangerous.